Spiral antenna array with polarization adjustment



$EARILII huuwn l5vu. ROTATION DEVICE INVENTOR ARTH U R E. MA RSTONJULIUS A. KAISER,JR.

ATTORNEY A. E. MARSTON ETAL Filed Nov. 28, 1958 RADI O y ENERGY DEVICESPIRAL ANTENNA ARRAY WITH POLARIZATION ADJUSTMENT LET-l- I90. ROTATIONSept. 18, 1962 DEVICE United States The invention described herein maybe manufactured and used by or for the Government of the United Statesof America for governmental purposes without the payment of anyroyalties thereon or therefor.

This invention relates to antenna systems in general and in particularto spiral antennas.

It is an object of the present invention to provide an antenna systemwherein the antenna pattern in the far field can be readily varied.

Another object of the present invention is to provide an antenna systemwherein the polarization in the far field can be readily varied.

Another object of the present invention is to provide an antenna systemwherein the far field polarization can be changed from linear tocircular by simple adjustments.

Other objects and many of the attendant advantages of this inventionwill be readily appreciated as the same becomes better understood byreference to the following detailed description when considered inconnection with the accompanying drawings wherein:

FIG. 1 shows a spiral antenna.

FIG. 2 shows a simple parasitic array involving spiral antenna elementsand a reflector.

FIG. 3 shows a more elaborate array employing several parasiticelements.

In accordance with the basic teachings of the present invention, anantenna system is provided wherein precise control over the polarizationand phasing of radiation at any point in the far field is possible. Withthis antenna system it is possible to obtain either circular or linearpolarization or any intermediate thereof in the far iield EIII and thedirection of linear polarization if such is used is readilycontrollable. Complex systems involving the principles of the presentinvention may be constructed which provide a wide range of antennapatterns from such diverse conditions as a narrow pencil beam in themajor axis of directivity to a conical pattern having a null in themajor axis of directivity. The foregoing flexibility of operation andresult is brought about by employing a plurality of spiral antennaelements, one or more thereof being a parasite, disposed substantiallyparallel to a driven spiral antenna element but spaced therefrom alongthe major aXis of directivity very much the same as the various elementsof a linear array of the yagi type are displaced relative to the drivenelement. Such an array of driven and parasitic spiral antennas can becaused to have various radition polarization characteristics dependingupon the combinations of configuration sense of individual elements andtheir spacings relative to each other and to the driven element.

A spiral antenna as typified in FIG. 1 is a planar assembly consistingof two or more interspaced conductors disposed layer upon layer insuch,a manner as to present a spiral configuration having a first or asecond sense depending upon whether the outward spiral from the centeris in a clockwise or counter-clockwise direction. For example, the twoconductors could be printed circuit conductors on a base member or discof insulating material. Each conductor has a starting point near thecenter of the disc and a termination at the periphery of the disc, theterminations of the two conductors occurring at diametrically opposedportions of the periphics ery. Such a spiral antenna may be energized atthe center by means of a coaxial cable with one conductor of the coaxialcable connected to one conductor of the spiral and the other conductorof the coaxial cab e connected to the second conductor of the spiral. Insuch a situation, a balanced to unbalanced converter feed is normallypreferable. When such a spiral is energized by radio frequency energy itradiates a broad circularly polarized beam to each side of the plane ofthe spiral. Each radiated beam is normal to the plane of the spiral andthe sense of circularity of polarization of the beam on any one sidecorresponds to the sense of the spiral as viewed from the opposite side.Accordingly, the two radiated beams are identical except that therotational sense of polarization of the radiated field on one side isthe opposite of that on the other. In many applications such as withparasitic elements as in the present invention is it desirable that thespiral radiate to one side only, such being readily accomplished byappropriately backing the spiral on one side with a ground plane, orwith a cavity. Where a ground plane is used, it is normally preferableto space the spiral antenna element and the ground plane apart by adistance equal to M4 or odd multiple thereof.

Although the exact theory of operation of such a spiral antenna is notuniversally established at the present time, a possible explanation isthat the spiral antenna behaves as if it were a two wire transmissionline which gradually by virtue of its spiral geometry transforms itselfinto a radiating structure or antenna. Ordinarily a two wiretransmission line wherein the wire spacing is a small fraction of awavelength yields a negligible amount of radiation when excited at itsterminals. This is due to the fact that the currents in the two wires ofthe line at any normal cross-section are out of phase so that theradiation from one line is essentially cancelled by the radiation fromthe other. In such an antenna as that shown in FIG. 1, if the spacingbetween adjacent wires is substantially smaller than the radius of theouter turn of the spiral, the diiference in length between the twoconductors from the origin to a point in the outermost circle isapproximately equal to half the circumference of the spiral. Withanti-phase excitation of the spiral antenna conductors at the center,the phasing gradually changes along the length of the two conductorsproceeding outwardly so that when the radius of the outer conductor is)x/Z'n' the currents in the two conductors are precisely in phase andradiation is at a maximum. Such a spiral antenna when excited at higherfrequencies where in the outer conductor radius is greater than\/2-nwould achieve such an in-phase condition at a smaller radius thanthe periphery so that portions of the conductors located at the smallerradius produce maximum radiation. Such an antenna thus is characterizedby wide band operation with respect to frequency because selectedportions thereof become effective at diiferent portions of the frequencyband.

The spiral antenna basically provides circularly polarized radiation.With a uniformly constructed antenna of the configuration of FIG. 1,there is circular symmetry of the radiation polarization about thespiral axis. Such symmetry allows rotation of the spiral antenna aboutits axis normal to the plane of the spiral producing a change in thephase of the radiated field everywhere in space without producingvariations in the amplitude of the signal in the far field. In suchrotation, one degree of mechanical rotation of the spiral antenna aboutan axis normal to the plane of the drawing of FIG. 1 produces a changeof phase in the far field of one electrical degree.

FIG. 2 shows a basic parasitic array embodying the principles of thepresent invention in which two spiral antenna elements 10 and 11 areplaced in front of a refiector '12. Each of the spiral antenna elements10 and 11 is similar to the basic spiral indicated in FIG. 1, that is,each has two conductors arranged in a spiral outward from a centralstarting point to a large radius. In each antenna the two conductors arerelatively insulated from each other and are held in position by somesuitable means, as an example in certain instances it may be desirableto employ a structure formed by printed circuit techniques namely thatwherein the two conductors are disposed on a suitable backing member ofinsulating material. Typically the first spiral antenna element '10 isconnected to an electrical lead such as a coaxial cable at its centerpoint. This coaxial cable may be typically surficiently rigid to providesuport for the spiral antenna element 10 in front of the reflector 12.Additionally the co axial cable 13 may contain an extension portion 14for supporting the second spiral antenna element 11in proximity to thefirst element 10. The coaxial cable 13 is connected by suitable lead-into a radio energy operative device 15 which may typically be atransmitter or a re ceiver or both connected to utilize energyintercepted by the antenna array or to radiate energy to a distantreceiver. The spiral antenna 10 radiates essentially only in a forwarddirection by virtue of its location in front of the reflector 12. Thespiral antenna element 11 is excited totally by energy coupled to itfrom the first spiral antenna element 10, there being no direct feed.Thus the sec-nd spiral is parasitically driven by the first. In thisarrangement, it is important that the side of the parasitic spiralantenna element 11 which faces the driven antenna element .10 have thesame configuration sense as the side of the driven spiral antennaelement which faces element 11. Accordingly, in this arrangement theforward radiating face of the parasitic spiral has a configuration sensewhich is opposite to that of the forward face of the driven spiralantenna element 10. This arrangement of faces insures that energy can becoupled to the parasitic spiral antenna element 11. The field resultingfrom this configuration will be a union of the driven field of element10 with the field from the parasite antenna element 11 and since both ofthese fields are circularly polarized but of opposite sense, thecomposite or sum field will in general be basically ellipticallypolarized, depending upon the amount of energy that is coupled to theparasitic spiral antenna element and reradiated. The amount of energycoupled from the driven spiral antenna element 10 to the parasiticspiral antenna element 11 is essentially a function of the spacingbetween the two spirals. In prac tice a variation in this coupling canbe achieved as facilitated by a slip joint device 14a which produces farfield polarization ranging from very near linear (-for 50% coupling) tocircular (no coupling between spiral elements).

A spiral antenna such as that of FIG. 2 has the following importantproperties; first if the parasitic element =11 alone is rotated throughan angle of 0 degrees, then the major axis of the ellipse ofpolarization for the radiated field on-axis will rotate through the sameangle 0. Second, if the parasitic element 11 is held stationary and thedriven element 10 is rotated through an angle 0 then the polarizationellipse of the far field remains stationary, but the phase of the energyin the far field experiences a change, which in the case of a linearlypolarized far field is in the amount of 0 electrical degrees.

Thus in the arrangement of FIG. 2 including a driving spiral antennaelement and a single spiral parasite it is possible to have essentiallylinear polarization of the far field, where the direction ofpolarization of the far field and the phase of the far field can beadjusted at will by an appropriate rotation of the constituent spiralantenna elements as indicated above. This rotation is provided byrotation device a which permits independent or coordinated rotation ofthe elements 10 and 11.

The apparatus of FIG. 3 is similar to that of FIG. 2 differing primarilyin the number of parasitic elements placed in front of the driven spiralantenna element 16. Element .16 is mounted in front of reflector 17 andis driven by means of coaxial line 18 from the radio energy device 19.In front of antenna elements 16 and suitably mounted are the spiralantenna parasite elements 20, 21 and 22. As before these elements areall disposed in such manner as to have similar configuration sensesfacing each other. Thus as viewed from the end of the array looking atthe element 22, spiral elements 16 and 21 will have one configurationsense Whereas the elements 20 and 22 will have the oppositeconfiguration sense.

In the above apparatus of FIG. 3 the driven spiral antenna element .16radiates energy forward into the linear array of parasitic elements andthis energy is coupled forward from one parasitic element to the nextwith the amount of coupling obtained between any two parasites being amatter essentially of the spacing between the two parasites. Bysuccessive application to this array of the principles set forth inconnection with FIG. 2 for control of phasing and polarization in thefar field by rotation of the antenna elements, it is possible to controlthe phase of the energy radiated from the individual elements of theparasitic array and so control the radiation patterns of the arrayitself. Thus both pencil beam patterns and conical patterns with a nullon axis can be achieved in this arrangement by appropriate spacing andangular orientation of the several parasitic spirals, facilitated byrotational device 19a and slip devices 14a.

Typical dimensions for a device of FIG. 2 are as follows: 1

Spiral antenna element diameter 4 in. Spacing of spiral antenna element10 from ground plane 2 in. Spacing of elements 10 and 11 forsubstantially linear polarization M32 Spacing of elements 10 and 11 forsubstantially circular polarization 3)\ Frequency of operation 1430 me.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed is:

1. An antenna system comprising, first and second two conductor spiralantenna elements each having direct space coupling, means supportingsaid antenna elements in proximity to each other in substantiallyparallel planes and with the same configuration sense each as seen fromthe other whereby there is coupling therebetween, and means coupling oneof said elements to a radio frequency operative device, the coupling ofthe other element to the radio frequency operative device being indirectthrough the coupling between the elements.

2. An antenna system comprising first and second two conductor spiralantenna elements in parallel planes with the antenna element centers ona selected axis each of said antenna elements having direct spacecoupling, the antenna elements having the same configuration sense asviewed one from the other, and means connecting one of said antennaelements to a radio frequency operative device.

3. An antenna system comprising, first and second two conductor spiralantenna elements, means supporting said antenna elements in parallelplanes with the antenna element centers on a selected axis perpendicularto the planes, the antenna elements having the same configuration senseas viewed one from the other, rotation control means for rotating atleast one of said elements about the selected axis and means connectingone of said antenna elements to a radio frequency operative device.

4. An antenna system comprising first and second spiral antennaelements, means supporting said antenna elements in parallel planes withthe antenna element centers on a selected axis perpendicular to theplanes, the antenna elements having the same configuration sense asviewed one from the other, means cooperative with said last named meansfor providing adjustment of the spacing of said antenna elements alongthe selected axis, rotation control means for rotating at least one ofsaid elements about the selected axis, and means connecting one of saidantenna elements to a radio frequency operative device.

5. An antenna system comprising first and second spiral antennaelements, means supporting said antenna elements in parallel planes withthe antenna element centers on a selected axis perpendicular to theplanes, the antenna elements having the same configuration sense asviewed one from the other, means cooperative with said last named meansfor providing adjustment of the spacing of said antenna elements alongthe selected axis, rotation control means for rotating at least one ofsaid elements about the selected axis, means connecting one of saidantenna elements to a radio frequency operative device and meansdisposed in proximity to said spiral antenna element for limiting thespace radiation pattern of the antenna system to one side of theparallel planes.

6. An antenna system comprising a plurality of spiral antenna elements,means supporting said antenna ele ments in separate parallel planes withthe antenna element centers on a selected axis perpendicular to theplanes, adjacent antenna elements having the same configuration sense asviewed from one to the other, means cooperative with said last namedmeans for providing adjustment of the spacings of said antenna elementsalong the selected axis, rotation control means for rotating at leastone of said elements about the selected axis, means connecting one ofsaid antenna elements to a radio frequency operative device, and areflector disposed in prox imity to said spiral antenna elements forlimiting the space radiation pattern of the antenna system to one sideof the parallel planes.

References Cited in the file of this patent UNITED STATES PATENTS1,342,306 Steinberger et a1. June 1, 1920 2,349,976 Matsudairi May 30,1944 2,656,839 Howard Oct. 27, 1953 2,773,254 Engelmann Dec. 4, 19562,856,605 Jacobsen Oct. 14, 1958 2,863,145 Turner Dec. 2, 1958 OTHERREFERENCES Glasgow, R. 5.: Principles of Radio Engineering, firstedition, 1936, McGraw-Hill Book Co., Inc., New York,

25 pages 450-453, and 461.

